DESCRIPTION: (Investigator's abstract) The long-term goal of this study is to understand the function of protein ubiquitination in hematopoiesis. Hematopoiesis is a complex process of cell proliferation and differentiation; dysregulated proliferation or a block in differentiation which results in leukemia. Regulation of hematopoiesis occurs at multiple levels including transcription factor expression, gene regulation, signal transduction and protein modification. Like phosphorylation and dephosphorylation, ubiquitination and deubiquitination are mechanisms of protein modification. The study of the biological function of ubiquitination and deubiquitination is far behind the study of protein phosphorylation. Two major groups of enzymes, ubiquitin conjugating enzymes and deubiquitinating enzymes, regulate the balance of protein ubiquitination. Although little is known about the biological function of protein ubiquitination in hematopoiesis and leukemogenesis, there is increasing evidence demonstrating the importance of this modification in numerous cellular events. We cloned a novel member of the deubiquitinating enzyme family, termed ubp46, from AML1-ETO knock-in mice. AML1-ETO fusion protein is generated from t(8;21), a common chromosomal translocation associated with acute myeloid leukemia (AML). The abnormal expression of ubp46 is associated with impaired hematopoiesis in both the yolk sac and fetal liver of these knock-in mice. Ubp46 expression pattern in normal adult mice and in hematopoietic cell lines strongly suggests that ubp46 plays an important role in hematopoiesis. Furthermore, our data from cell line analysis indicate that overexpression of ubp46 blocks the terminal differentiation of monocytic cells similar to what is observed in myeloid leukemia, suggesting a potential role of ubp46 in pathogenesis of AML. Therefore, the overall goal of this proposal is to study the function of a novel deubiquitinating enzyme, ubp46, in hematopoietic cell proliferation, lineage commitment, and differentiation. The specific aims are: to study ubp46 function by expressing it in hematopoietic cells, to identify the partners or substrates of ubp46 through protein-protein interaction analyses, and to analyze the effect of ubp46 on hematopoiesis by generating ubp46 knockout ES cells and knockout mice.